Probably the shed coming loose in the nasal passage. If you hold him in your hand and can feel the gurgling in his lungs there is a problem. But whistling is most likely loose skin.
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check his mouth, make sure there is no mucus, cause that might be a sign on an RI, keep a close eye on him, this is how i figured out mine had an RI and i was able to fix it on time, if he keeps doing it for more than a couple of days i would advice you to get him checked out, better safe than sorry, other than that it might just be him going into shed, just watch him closely =]